During the spring of 2004, corn seedlings with symptoms of wilting and stunting were observed in corn fields with emergence problems in Madison and Brown counties, Ohio. Phytophthora isolates were recovered from sections of root tissue of diseased seedlings placed on dilute V8 media amended with pentachloronitrobenzene, iprodione, benlate, neomycin sulfate, and chloramphenicol. Colonies were rosaceous on potato dextrose agar, with a growth rate of 5 mm per day. Homothallic isolates with paragynous antheridia were observed on lima bean agar (LBA); oogonia were 35 to 50 μm in diameter. Sporangia were ovoid to obpyriform, nonpapillate, with an average size of 49 × 30 μm. Pathogenicity was tested on corn seeds using a petri dish assay with 3-day-old cultures on LBA and a sand-cornmeal cup test amended with inoculum from 7-day-old cultures on LBA (1). After 1 week in the petri dish assay, the seeds failed to germinate completely and were covered with white, fungal-like, aerial mycelia and the pathogen was recovered from brown discolored radicle roots. In the cup assay, 2-week-old seedlings developed the same symptoms observed in the field; the pathogen was also isolated from brown discolored roots. In both assays, no symptoms developed in the noninoculated controls. Both pathogenicity tests were repeated two times. Genomic DNA was extracted from mycelia of two isolates and the internal transcribed spacer (ITS) region was amplified and sequenced using ITS6/ITS4 primers (2). Both isolates had identical ITS sequences (GenBank Accession No. GQ853880). A BLAST search of the NCBI database showed 100% homology with the sequence of the haplotype isolate of Phytophthora sansomeana (Accession No. EU925375). P. sansomeana is a new species characterized principally by a large oogonial diameter (37 to 45 μm), rapid growth rate (7 to 10 mm/day), and an ITS sequence falling in Cooke's clade 8 (4). Pathogenicity tests, morphological characteristics, and the ITS sequence analysis indicate that P. samsomena is the causal agent of the symptoms observed on corn seedlings. P. sansomeana has been reported as a pathogen of soybean in Indiana, Douglas-fir in Oregon, and weeds in alfalfa fields in New York (4). To our knowledge, this is the first report of P. sansomeana infecting corn in Ohio, albeit other isolates have previously been recovered from soybean in the state. There are four previous reports of Phytophthora spp. affecting corn in the United States and Mexico (3). Crop rotation will have little effect in management of this pathogen since corn and soybean are produced in the same fields continuously throughout the state.
References: (1) K. E. Broders et al. Plant. Dis. 91:727, 2007. (2) D. E. L. Cooke et al. Fungal Genet. Biol. 30:17, 2000. (3) D. F. Farr et al. Fungi on Plants and Plant Products in the United States. The American Phytopathological Society, St. Paul, MN. 1989. (4) E. M. Hansen et al. Mycologia 101:129, 2009.
